1. Field of Invention
This invention generally relates to electrostatic printing apparatus and methods of electrostatic printing. In particular, the invention relates to methods and apparatus for enhancing the image resolution characteristics of an electrostatic printing apparatus, especially when performing image-next to-image (INI) printing processes.
2. Description of Related Art
Electrostatographic printing is well known and commonly used for copying or printing documents on a paper substrate. Electrostatographic printing is performed by forming a substantially uniform charge on a photoconductive member and exposing the photoconductive member to a pattern of light. Exposing the photoconductive member to the pattern of light generates a corresponding electrostatic latent image on the photoconductive member. Toner particles are then deposited onto the photoconductive member so that the toner particles are selectively deposited in either charged or discharged areas on the photoconductive member. The developed toner image is then typically transferred to a substrate and fixed to the substrate by heat and/or pressure. The photoreceptor is then cleaned of any residual toner or electric charge in preparation for another charge/electrostatic latent image generating/development process.
For the purposes of the present description, the concept for latent image development via direct surface-to-surface transfer of a toner layer via image-wise forces will be identified generally as Contact Electrostatic Printing (CEP). As one variant of CEP, a thin layer of liquid developer is brought into contact with an electrostatic latent image on another surface, wherein development of the latent image occurs upon separation of the first and second surfaces, as a function of the electric field strength generated by the latent image. In this process, toner particle migration or electrophoresis is replaced by direct surface-to-surface transfer of a toner layer induced by image-wise fields.
Patents which may describe certain general aspects of CEP, as well as specific apparatus therefor, may be found in U.S. Pat. Nos. 5,436,706 and 5,596,396, issued to Landa et al., U.S. Pat. No. 5,619,313, issued to Domoto et al., as well as other patents cited therein.
Electrostatographic printing systems that perform image-next to-image (INI) printing processes are known. In an INI process, also known as highlight or spot color processes, toner layers are arranged side-by-side to achieve a desired image. For example, a highlight electrostatographic printing apparatus can form an image having a blue (the highlight color) square positioned next to a black square by forming two toner layers of black and blue toner next to each other.
However, image quality problems exist with the known printing systems when two images of different colors are developed onto a single image bearing member. In the known systems, the two different toners are typically deposited at two different times, often separated by a photoreceptor recharge and electrostatic latent image generation process. If the toner constituting the first image is still active (charged and can move under an electric field) when going through the development step of the second color, the toner can be removed from the image bearing member by the electrostatic field corresponding to the second latent image. In particular, the toner of the first image in the background area of the second image will be removed and give rise to the difficulties of producing two color images next to each other. Thus, the first layer of toner deposited on the photoreceptor is often attracted to and displaced when the second electrostatic latent image is formed on the photoreceptor and developed.
For example, FIG. 1 shows an intended toner image to be formed on a photoreceptor 1. In FIG. 2, a first development process is performed using first toner particles 2. In this example, charged area development (CAD) is performed with positive polarity first toner 2. In CAD, the discharged areas 3 of the photoreceptor (denoted by "o") have substantially no charge or have little charge so that the first toner particles 2 are not attracted to the discharged areas 3.
FIG. 3 shows the photoreceptor during second development using second toner particles 4. The second toner particles 4 are shown as squares in FIG. 3 to more clearly indicate the different toner particles. However, the second toner particles 4 do not necessarily have a square shape. In some cases, the first toner particles 2 are attracted away from the photoreceptor 1 due to charge on the toner support 5 or other electrostatic forces. FIG. 4 shows the photoreceptor 1 after the second development process using second toner particles 4 is complete.